The disclosure of Japanese Patent Application No. 2001-162549 filed on May 30, 2001, including the specification, drawings and abstract, is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to a planetary gear set for use in various power transmission apparatuses such as automatic transmissions and the like. More particularly, the invention relates to a structure of a carrier that supports pinion gears of the planetary gear set and to a method of manufacture of the carrier.
2. Description of the Related Art
A planetary carrier in a planetary gear set is an element that transfers rotation of pinion gears that rotate meshes with a sun gear and a ring gear, as is well known. The pinion gears are rotatably supported on the planetary carrier by pinion shafts that are fixed on the planetary carrier via bushes or the like. Pinion shaft-supporting portions of the planetary carrier undergo torsional deformation caused by loads associated with the power transfer via the pinion gears, so that friction occurs due to minor sliding between peripheral surfaces of the shafts and the wall surfaces of holes in which the shafts are supported. To attain good durability and to withstand the aforementioned friction, planetary carriers are conventionally produced by machining hot-forged workpieces although the hot-forged pieces have higher hardness and therefore are more difficult to work than cold-forged workpieces and pressed workpieces. Moreover, the use of a hot-forged workpiece as a material of a planetary carrier leads to a considerably higher production cost due to low productivity and poor stock utilization in machining.
However, other problems are posed by fabrication of a planetary carrier from a cold-forged workpiece that offers good workability and good stock utilization, or from a pressed workpiece that provides higher productivity and, more specifically, by the quench-hardening of only the shaft-supporting portions of such workpieces that require abrasion resistance. Because, the thermal treatment is performed after finishing of the shaft-supporting portions, which normally involves boring followed by reaming, due to requirements for high positional precision and parallelism, a conventional quenching process with a conventional heating depth causes considerable thermal strain in the shaft-supporting portions. This problem is particularly acute in the case of pressed workpieces due to their high susceptibility to thermal strain.
Accordingly, it is an object of the invention to improve the durability of support portions of a planetary carrier that support pinion gears of a planetary gear via pinion shafts by adopting a quenching process that has a reduced thermal effect.
Another object of the invention is to further reduce the thermal affect of quenching on the support portions by providing a directional quenching.
The aforementioned objects are achieved by the present invention which provides a planetary carrier which supports pinion gears of a planetary gear set via pinion shafts and which has, in a pinion shaft-supporting portion, a shallow hardened surface portion produced by induction quenching and a method of manufacture of same.
Accordingly, the present invention provides a planetary carrier for supporting a pinion gear of a planetary gear set via a pinion shaft and has a support portion which supports the pinion shaft, the support portion having a shallow hardened surface produced by localized induction heating and quenching. In the preferred embodiment described below, the support portion includes a shaft hole into which the pinion shaft is fitted and the hardened surface is at least a portion of a peripheral wall surface defining the shaft hole.
The present invention further provides a method of quench hardening a support portion of a carrier for a planetary gear set wherein the support portion has a hole which receives a pinion shaft. The method includes inserting an induction coil into the hole and supplying high frequency current to the induction coil to inductively heat a peripheral wall surface surrounding the hole. Heating with a high frequency current to the induction coil of at least 400 KHz allows for subsequent quenching of the heated wall surface to produce a uniquely shallow hardened surface portion surrounding the hole. In one embodiment, the heating is performed with the support portion and the induction coil immersed in water. In another preferred aspect of the method of the present invention the induction coil is arc-shaped with an insulated slit and wherein a portion of the peripheral wall surface defining the hole, which is most remote from the insulated slit, is preferentially heated and hardened. The method may further include the pressing of a steel plate to form a carrier portion and the drilling and reaming of the pinion shaft receiving holes in the pressed plate.